Noise reduction tubes

ABSTRACT

The invention uses tubes for the reduction of radiated duct noise. The tubes may be used to attenuate narrow band noise by diverting a portion of the acoustic energy through the tubes back into the duct. Upon reintroducing the diverted flow out of phase with the primary flow, acoustic cancellation is achieved. The frequency at which this occurs is dependent on the difference in the path lengths through the tubes and the separation distance—the distance separating the inlet and outlet of the tube. This invention can be used over varying frequency ranges by using an arrangement of flexible, constant length tubes. The effective frequency range is tailored by varying the separation distance and also allows the varying the angular arrangement of the tubes for more effective attenuation of spinning modes having varying propagation angle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applicationNo. 60/554,269, filed Mar. 18, 2004, the contents of which areincorporated in their entirety.

BACKGROUND

A noise reduction system is often used to reduce noise energy in a ductor duct-like device due to some noise source. Such a noise source may bedue to, but not limited to, the turbofan of an aircraft engine. Over thelast several decades, much work has been done to attenuate noisegenerated by aircraft engines.

There are currently two available alternative technologies for reducinginlet noise in jet engines. One technology simply employs “liners” onthe engine compartment which are internal coatings that absorb acousticenergy at the engine inlet. This technology is very limited in that itdoes not reduce noise over a large frequency range, but is mainlylimited to broadband noise. Also, liners become ineffective with timebecause of changes in material properties due to accumulation of dirt,dust and liquids in the absorptive material. Also, the sound reductionobtained from liners is limited since the amount of reduction isdirectly proportional to the amount of surface treatment. Thus, if anoperator wants to greatly reduce the noise using the liner, the operatormust use more liner material over a larger surface area. This addsunwanted weight to the aircraft, which affects the fuel consumption ofthe aircraft.

Additionally, there are active noise cancellation systems known ascompression type acoustic drivers, which are effective at specificfrequencies. Unfortunately these devices are heavy and expensive and arenot durable; i.e., the poor reliability of the moving parts would have anegative impact when used in commercial engines. Furthermore, theelectrical power requirement to drive these compression drivers is muchtoo great.

Fan noise has also been identified as a major technical concern in thedevelopment of the future engines. Future engines such as an ultra-highbypass (UHB) engine has great fuel efficiency, but at the cost of a highnoise level. The introduction of ultra high bypass ratio engines havingshorter inlet ducts relative to the size of the fan lessens theeffectiveness of passive acoustic liners because as the frequenciesdecrease the acoustic wavelength increases.

Previous experiments using circumferential arrays of tubes orientedparallel to the inlet duct axis were successful. Power attenuation of upto 8 dB was achieved with the added benefit of 3 dB broadband powerreduction of up to 3200 Hz. Theoretical analyses further showed thecapabilities of the use of tubes angled to coincide with the propagationangle of the disturbing wave. One study showed, for a wave at 2150 Hzwith a propagation angle of 40° from the duct axis, a fixedcircumferential array of rigid Herschel-Quincke tubes arranged tocoincide with the propagation resulted in 4.1 dB of power reductioncompared to just 2.7 dB using an array of tubes oriented parallel to theduct axis.

Previous embodiments of Herschel-Quincke tube treatments have been usedsuccessfully to combat turbofan noise due to an engine running atconstant speed. However, these treatments could not account forvariations in frequency content and variations of the angle ofpropagation of the disturbance. U.S. Pat. No. 6,112,514 to Burdisso etal., the contents of which are incorporated in their entirety describesa system with Herschel-Quincke tubes to reduce frequencies at a steadystate operation. U.S. patent application Ser. No. 10/343,567 filed Oct.2, 2001 to Byrne et al, the contents of which are incorporated in itsentirety, is an improvement over the Burdisso '514 patent in allowingfor more adjustment to dynamic operating conditions. For years it wasdesired to still further noise from such apparatus as jet engines duringtransition periods of takeoff or landing when closest to populationcenters than the Burdisso and Byrne teachings provide. The instantinvention addresses the problems of the prior art during acoustic noisepollution generated during transition phases that were not addressed byprior attempts to reduce noise.

SUMMARY OF THE INVENTION

This invention is intended for the reduction of sound propagatingthrough a duct. The invention allows the tuning of the acoustictreatment used to reduce the sound propagation by accounting for changesin the frequency and propagation angle of the troublesome sound waves.The invention reduces unwanted noise propagating in a duct shaped deviceby attenuation by diverting a portion of the acoustic energy andreintroducing it out of phase up stream. The phase reversal isaccomplished due to the difference in the path length between theprimary and secondary flow paths (FIG. 1.). This invention uses tubesmade instead from a flexible material (such as but not limited topolymer hoses) that allows for varying the separation distance as wellas the angular orientation. These embodiments discussed below areapplicable to all ducts and most especially to a turbofan aircraftengine having a duct like housing.

Variation in the separation distance, while maintaining a fixed lengthfor the secondary acoustic path through the tubes, allows for theattenuation of noise over varying frequency ranges. The flexible designalso allows the angular orientation of the tubes to be changed such thatthe tubes may be aligned with the propagation angle of a spinningacoustic mode within the duct. Further, the orientation angle of thetubes may be varied as the propagation angle of the spinning mode variesthus maximizing the attenuation of the spinning mode. Separationdistance and angular orientation may be also be changed in variouscombinations allowing the acoustic treatment to be tuned to optimallyreduce sound from spinning modes having varying frequency content andangular orientation.

One embodiment of the noise attenuation apparatus for ducts such asturbofan aircraft engines comprises at least one tube having an inletend and an outlet end, wherein the ends are separated by a distancecapable of being varied during operation, wherein the ends are in fluidcommunication with the duct. A distance actuator for changing thedistance between the inlet end and the outlet end of the tube within theduct allows for dynamic tuning of the tubes by repositioning the tubeends in the fixed length flexible tubes.

Another embodiment includes an orientation actuator, wherein the tubehas an orientation angle that can be changed by the orientationactuator. This in combination with the adjustment of the tube endsdynamically reduces unwanted noise in ducts.

Another embodiment includes a control system to control the separationdistance of the at least one tube. The embodiment may include an outlettuning ring capable of rotation around the axis of the duct and movementalong the length of the duct. The position of the inlet end of the tubemay also be fixed relative to the duct. The position of the outlet endof the tube may also be fixed relative to the duct. The distance betweenthe inlet end of the tube and the outlet end of the tube on the duct mayboth be moved relative to the duct when the distance between the twoends are changed.

In one embodiment an outlet tuning ring can be capable of rotationaround the axis of the duct and movement along the length of the ductwherein the inlet end fixed relative to the duct and the outlet end ofthe tube is affixed to the outlet tuning ring. Another embodiment mayinclude an inlet tuning ring capable of rotation around the axis of theduct and movement along the length of the duct wherein the outlet end ofthe at least one tube is fixed relative to the duct and the inlet end ofthe tube is affixed to the inlet tuning ring.

In another embodiment an inlet tuning ring capable of rotation aroundthe axis of the duct and movement along the length of the duct whereinthe inlet end of the at least one tube is attached to the inlet tuningring; and, an outlet tuning ring capable of rotation around the axis ofthe duct and movement along the length of the duct wherein the outletend of at least one tube is attached to the inlet tuning ring.

Another embodiment of a noise attenuation apparatus for ducts such asturbofan aircraft engines comprises at least one tube having an inletend and an outlet end, wherein the ends are separated by a distancecapable of being varied during operation, wherein the ends are in fluidcommunication with the duct. Furthermore, at least one flexible tubebranch attached in fluid communication to at least one end of the atleast one tube. There are at least three ports wherein the at leastthree ports comprises an inlet port at the inlet end of the at least onetube having an open position that allows the inlet end of the tube to bein fluid communication with the duct and a closed position to preventthe inlet end to be in fluid communication with the duct. Also includedis a branch port that controls fluid communication between the at leastone tube and the at least one branch tube. Lastly an outlet port at theoutlet end of the at least one tube having an open position that allowsthe inlet end of the tube to be in fluid communication with the duct anda closed position to prevent the inlet end to be in fluid communicationwith the duct. The apparatus may include a measurement system capable ofsensing the presence of spinning and non-spinning acoustic modes andtheir respective amplitudes, phase, resonant frequencies and angles ofpropagation. A control system may be used to control the fluidcommunication of the at least one tube with the duct. Further optionsinclude an outlet tuning ring capable of rotation around the axis of theduct and movement along the length of the duct. The embodiment caninclude an outlet tuning ring capable of rotation around the axis of theduct and movement along the length of the duct wherein the inlet endfixed relative to the duct and the outlet end of the tube is affixed tothe outlet tuning ring.

A method of noise attenuation for ducts such as turbofan aircraftengines comprising the steps of providing at least one tube having aninlet end and an outlet end, wherein the ends are separated by adistance capable of being varied during operation, wherein the ends arein fluid communication with the duct and the tube has a fixed length.The next step is measuring the frequency of the sound waves of the duct.Then actuating with a distance actuator for changing the distancebetween the inlet end and the outlet end of the tube within the duct toreduce noise. The noise may be reduced further by moving at least onetuning ring capable of rotation around the axis of the duct and movementalong the length of the duct wherein the end of the at least one tube isattached to the inlet tuning ring to reduce noise. The method caninclude providing a turbofan aircraft engine having a duct.

It is therefore an object of the present invention to provide anadjustable noise reduction system to reduce both broadband and tone fannoise components through a range of frequencies.

It is another object of the present invention to provide a noisereduction system that reduces noise at both inlet and outlet ports of anoise generating system, and more specifically turbofan engines atdifferent engine speeds.

The invention is directed to the attenuation of inlet and outlet noisefrom turbofan engines. The present invention utilizes an array ofspecially designed fixed length tubes to effectively divide the acousticenergy generated by the engine. One of the energy components propagateswithin the tubes while the other propagates within the enginecompartment. At some time certain, the acoustic energy in the tubes isreintroduced into the engine compartment to cancel the acoustic energyremaining in the engine as it propagates from the fan towards the inletand outlet openings.

Several arrays of tubes may be used on one duct or duct-like structure.Such an arrangement would thus provide attenuation of several acousticmodes each having different ranges of frequency content and angularorientation. For such an arrangement, each array may have tubes ofdifferent lengths, cross-sectional areas and tube material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1. shows the prior art apparatus for reducing noise;

FIG. 2. shows an embodiment of the apparatus with a constant length,flexible tube mounted to a duct. The constant length, flexible tube isshown for two separation distances given a fixed inlet location and avariable outlet location;

FIG. 3. shows the path of an acoustic wave front of a spinning modepropagation through a round duct. The angle of propagation and the wavenumber components are defined;

FIG. 4. shows an embodiment of the apparatus with a constant length,flexible tube mounted to a duct. The constant length, flexible tubeshows two separation distances given a variable inlet location and afixed outlet location;

FIG. 5. shows an embodiment with a constant length, flexible tubemounted to a duct. The constant length, flexible tube is shown for twoseparation distances given both a variable inlet location and a variableoutlet location;

FIG. 6. shows an embodiment using a constant length, flexible tubemounted to a duct for frequency tracking. The constant length, flexibletube is shown for two separation distances such that the tube is tunedto attenuate the variable frequency acoustic mode at two differentfrequencies. A fixed inlet location and a variable outlet location isshown however the separation distance may be accomplished also using thevariable outlet location or the use of both a variable outlet and avariable inlet;

FIG. 7. shows an embodiment with the placement of a circumferentialarrangement of constant length, flexible tubes mounted to a duct. Thisarrangement is shown with the outlet tuning ring used to vary therotational orientation and separation distance of the tubes by changingthe axial and angular position of the outlets;

FIG. 8. shows an embodiment with the placement of a circumferentialarrangement of constant length, flexible tubes mounted to a duct. Thisarrangement is shown with the inlet tuning ring used to vary therotational orientation and separation distance of the tubes by changingthe axial and angular position of the inlets;

FIG. 9. shows an embodiment with the placement of a circumferentialarrangement of constant length, flexible tubes mounted to a duct. Thisarrangement is shown with both the inlet tuning ring and the outlettuning ring used to vary the rotational orientation and separationdistance of the tubes by changing the axial and angular position ofeither the outlets, the inlets or both simultaneously;

FIG. 10. shows the change in the orientation angle of a spinning modegiven a change in the resonant frequency of that spinning acoustic mode;

FIG. 11. shows the placement of several constant length, flexible tubeswith inlet and outlet tuning rings mounted on a turbofan aircraftengine;

FIG. 12 is a detail view of one flexible tube emphasizing the use andplacement of the inlet and outlet tuning rings and the mechanism bywhich the linear and angular orientations of the inlet and outlet tuningrings may be varied;

FIG. 13 shows the placement of a single array of constant length,flexible tubes mounted at the inlet of a turbofan engine;

FIG. 14 is a detail view of one flexible tube emphasizing the use andplacement of the inlet and outlet tuning rings and the mechanism bywhich the linear and angular orientations of the inlet and outlet tuningrings may be varied;

FIG. 15 shows the use of a branched, constant length, flexible tubewhere the branched portion is closed such that the remaining portionacts as a single constant length, flexible tube;

FIG. 16 shows the use of a branched, constant length, flexible tubewhere the branched portion is open and the remaining portion is closedsuch that the apparatus acts like a single constant length, flexibletube; and

FIG. 17 shows the use of a branched, constant length, flexible tubewhere all portions are open such that three acoustic paths existallowing three acoustic modes to be attenuated simultaneously.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a noise reduction system directed to reducingnoise in, duct like structures, especially turbofan engines. The noisereduction system of the present invention reduces noise energy over awide range of frequencies for both tonal and broadband components of theinlet and outlet noise for turbofan engines. The present inventionutilizes at least one fixed length tube assembly comprising at least onedynamically adaptable fixed length tube as described herein, in oneembodiment an array of such tubes may be arranged in a circumferentialor helical array about the turbofan engine to reduce the noise levelsgenerated. The assembly or assemblies may also be placed at the inlet,and in other locations, such as the upstream or downstream locationsfrom the turbofan engine. The inlet and outlet of the tubes of theassembly can be placed parallel to the engine axis or at an angle. Byattaching an array of dynamically adaptable fixed length tubes ofappropriate length onto the inlet and/or outlet of the turbofan engineit creates destructive waves that cancel the acoustic energy in theturbofan engine without increasing fuel consumption.

The present invention is used for the reduction of sound existing in orpropagating through a duct or duct-like structure. Acoustic modes withinlong round ducts may be described by their axial and radial wavenumbercomponents. The vector sum of these wavenumber components describes theacoustic wave having wavenumber, k_(cs), traveling at angle θ. This isdescribed in FIG. 3. and mathematically ask _(z) ² =k _(cs) ² −k _(s) ²  equation (1)where c is the circumferential mode index and s the radial mode index.Note that k_(s)=q/a where a is the duct radius and q the integer numberof wavelengths fitting around the circumference. The pressuredistribution is represented graphically for several combinations ofcircumferential and radial mode indices (q, p) in FIG. 2 where areas oflike color are in phase. Acoustic modes having q=0 propagate withoutspinning since k_(cs)=k_(z) resulting in a circumferential wavenumber,k_(c)=0 m⁻¹. This is emphasized in equation (2)k _(z) ² =k _(cs) ²−(q/a)²  equation (2)The helix angle or propagation angle, θ, may be determined knowing thewavenumber components as

$\begin{matrix}{\theta = {\tan^{- 1}\left( \frac{k_{z}}{k_{s}} \right)}} & {{equation}\mspace{14mu}(3)}\end{matrix}$with circumferential wavenumber, k_(s)=q/a and axial wavenumber

$\begin{matrix}{k_{z} = \frac{{- {Mk}_{0}} + \sqrt{k_{0}^{2} - {\left( {1 - M^{2}} \right)k_{qp}^{2}}}}{1 - M^{2}}} & {{equation}\mspace{14mu}(4)}\end{matrix}$where M is the flow Mach number. The acoustic wavenumber, k₀=ω/c for theduct mode of angular frequency, ω of phase speed, c has eigenvaluesk _(qp)=χ_(qp) /a  equation (5)where χ_(qp) are the inflection points of the Bessel function of thefirst kind of order q.

The frequency content of typical turbofan engine noise may be describedas broadband with distinct tonal responses at the blade passagefrequency (BPF) and harmonics. For subsonic aircraft, these tones aretypically 10-20 dB above the broadband response with the BPF responsemost often the loudest. The BPF and harmonics are generated by theinteraction of the turbofan rotor and stator blades. The BPF, in Hertz,may be calculated by

$\begin{matrix}{f_{BPF} = \frac{\Omega\; N_{B}}{60}} & {{equation}\mspace{14mu}(6)}\end{matrix}$where Ω is the angular speed of the rotor shaft in revolutions perminute and N_(B) the number of fan blades. The n^(th) harmonic of theBPF may be determined byf _(n) =nf _(BPF)  equation (7)where n=(1,2,3, . . . ).

The solution to the homogeneous wave equation, for waves inside a rigidduct generated by a turbofan, may be expressed as

$\begin{matrix}{{p\left( {r,\theta,z,t} \right)} = {\sum\limits_{q = 0}^{Q}\;{\sum\limits_{p = 0}^{P}{A_{qp}{{\cos\left( {{q\;\Psi} - {2\pi\;{nN}_{B}\Omega\;{t/60}} + \phi_{qp}} \right)}.\left\lbrack {{J_{q}\left( {k_{qp}r} \right)} - {\frac{J_{q}^{\prime}\left( {k_{qp}a} \right)}{Y_{q}^{\prime}\left( {k_{qp}a} \right)}{Y_{q}\left( {k_{qp}r} \right)}}} \right\rbrack}{\mathbb{e}}^{{- {\mathbb{i}}}\; k_{z,{pq}^{z}}}{\mathbb{e}}^{{\mathbb{i}}\; 2\pi\; f_{\beta\;{PF}^{t}}}}}}} & {{equation}\mspace{14mu}(8)}\end{matrix}$where t is time, A_(qp) the complex modal amplitude, k_(z,qp) the axialwavenumber for mode (q,p), J_(q)( ) the q^(th) order Bessel function ofthe first kind and Y_(q)( ) the q^(th) order Bessel function of thesecond kind. J_(q)′( ) and Y_(q)′( ) are, respectively, the derivativesof J_(q)( ) and Y_(q)( ).

The mode angular velocity may be expressed asΩ_(qp)=2πnN _(B)Ω/60q  equation (9)and for real values of the axial wavenumber, k_(z,qp), the (q,p) modewill propagate down the duct whereas for k_(z,qp) imaginary, the modedecays exponentially.

To understand the innovations represented by using Constant Length,Flexible Herschel-Quincke (HQ) Tubes, it is necessary to understandfirst the conventional implementation of the fixed length HQ treatment.Current technology requires the HQ tubes to be designed such that theymay attenuate a particular resonant frequency, although they have alsobeen reported to attenuate broadband acoustic power by up to 3 dB.Previous research efforts allowed no provision for adjustment of the HQtube properties. These treatments, then, must be designed to attenuatenoise generated for a particular condition. In particular, the designwould be for a specified, troublesome tone of a turbofan given aspecified engine speed—for instance, at the operating speed duringapproach.

The required difference in the two path lengths of the HQ tubes may bedescribed asΔl=(2m+1)(λ/2)  equation (10)where m=0,1,2,3, . . . andλ=c/f  equation (11)The minimum number of tubes required must be more than four times thehighest acoustic circumferential duct mode orN≧4q+1  equation (12)Since the HQ tube design method presented here assumes the acousticwaves traveling within them are plane waves, the HQ treatment is limitedby an upper frequencyf _(upper) =c/2S  equation (13)where S is the tube cross sectional area.The dimensions of the components of the acoustic treatment describedwithin the present invention specified herein are for exemplary purposesillustrating the details of one particular configuration. Thesedimensions may vary depending on the application and are not to beconsidered limitations of the present invention.

The acoustic treatment of the present invention uses at least one arrayof constant length, flexible tubes to attenuate the noise within theduct or duct-like structure. Several arrays of flexible tubes may bedesigned to be used simultaneously such that several acoustic modes,each of varying frequency content and propagation angle, may beattenuated.

In FIG. 1, a single Herschel-Quincke tube is used to demonstrate theprior art. A duct 100 and a single, rigid Herschel-Quincke tube 101 aremounted in parallel. The tube 101 has an inlet 102 and an outlet 103.The primary path of the acoustic mode 110 travels along the duct 100. Atthe inlet 102 a portion of the acoustic mode diverts into a secondarypath 111 into the tube 101 with the remaining acoustic energy 112traveling through the duct 100. The secondary path 111 is thenreintroduced into the duct 100 out of phase with 112 through the outlet103. The combination of the primary path 110 and the secondary path 111at the outlet 103 results in the attenuation of the acoustic mode 113.

The description of the current invention begins in FIG. 2. The duct 100has a constant length, flexible tube 201 mounted in parallel. In thisconfiguration, the tube 201 has an inlet 202 which is fixed in positionand an outlet shown in two different positions 203, 204. Theconfiguration having the outlet at position 203 has a separationdistance of S₁. The configuration having the outlet at position 204 hasa separation distanced of S₂. Note that in FIG. 2, S2>S1 and the lengthof the tube 201 is the same, L, for all separation distances. Oneembodiment of the noise attenuation apparatus for ducts such as turbofanaircraft engines comprises at least one tube 201 having an inlet end 202and an outlet end 203, wherein the ends are separated by a distance S1capable of being varied during operation to a second position S2,wherein the ends are in fluid communication with the duct. A distanceactuator 412, 422 for changing the distance between the inlet end 202and the outlet end 203 of the tube within the duct allows for dynamictuning of the tubes by repositioning the tube ends in the fixed lengthflexible tubes.

FIG. 3 shows a round duct 100 of radius r and the wave front 110 of aspinning acoustic mode. The wave front 110 propagates with wave numberk_(cs) and corresponding angle of propagation θ. The axial wavenumbercomponent is k_(z) and the radial wave number component k_(s).

FIG. 4 depicts a different embodiment that has a variable inletlocation, shown at two positions 210, 211 and a fixed outlet position212.

FIG. 5 depicts an embodiment that has a variable inlet location, shownat two positions 210, 211 and a variable outlet position 203, 204.

FIG. 6 depicts an embodiment of frequency tracking using a flexible,constant length tube. Assuming the tubes of this present invention aredesigned to have a path length difference of one-half wavelength (m=0),by equation (10) and equation (11), it is shown that a decrease infrequency requires an increase in separation distance in order for thetube to operate effectively. Note that while the separation distancechanges the tube arclength, L, which is the fixed or constant length ofthe flexible or compliant tube as described for the present invention,remains constant.

As an example, for a tone at 2000 Hz, the wavelength in air at sea level(c=343 m/s) would be 17.2 cm. By equation (10) the resulting minimumpath difference, for m=0, would be 8.6 cm. Therefore, the tube of thispresent invention may be designed having a centerline arclength, L withan inlet/outlet separation distance of L−8.6 cm.

Assuming the engine speed slows resulting in the tone decreasing to 1800Hz, the new required minimum path difference increases to 9.6 cm from8.6 cm. The tube is no longer ideally positioned to attenuate the tone.Shortening the inlet/outlet separation distance, by, for example, movingthe outlet of the present invention end 1.0 cm closer to the inlet,giving an inlet/outlet separation distance of L−9.6 cm, will tune thetube of this present invention to 1800 Hz allowing continual reductionof the tone as it changes frequency. Note that the inlet/outletseparation distance may be adjusted by moving either the inlet, theoutlet (as shown here) or both the inlet and the outlet as the enginechanges speed to allow for continuous noise reduction.

FIG. 7 depicts the wave front of a spinning mode 300 oriented at apropagation angle θ along the path 301. The use of an outlet tuning ring310 to which the outlets of an array of tubes 201 are attached is shownand oriented inline with 301. The outlet tuning ring 310 is capable ofrotational adjustment 311 of the tube outlets for spin rate tracking andaxial adjustment 312 of the tube outlets for frequency tracking.Combinations of the adjustments 311 and 312 may be performedsimultaneously such that the array of tubes may be adjusted as thepropagation angle and frequency of an acoustic mode changes.

FIG. 8 depicts an embodiment that has the inlets of the tubes 201attached to the inlet adjustment ring 320. Rotational adjustment 321 andaxial adjustment 322 may be simultaneously adjusted to account forchanges in the propagation angle and frequency of an acoustic mode.

FIG. 9 depicts an embodiment that uses both the outlet adjustment ring310 and the inlet adjustment ring 320. The two adjustment rings 310 and320 may each be adjusted to produce a relative separation distance and arelative orientation angle of each tube 201.

FIGS. 8-9 shows embodiments that include an orientation actuator 311,312, 321, 322, wherein the tube has an orientation angle that can bechanged by the orientation actuator 311, 312, 321, 322. This incombination with the adjustment of the tube ends dynamically reducesunwanted noise in ducts.

Embodiment in FIGS. 7 and 8 include a control system to control theseparation distance of the at least one tube. The embodiment may includean outlet tuning ring capable of rotation around the axis of the ductand movement along the length of the duct 311, 312, 321, 322. Theposition of the inlet end of the tube may also be fixed relative to theduct. The position of the outlet end of the tube may also be fixedrelative to the duct. The distance between the inlet end of the tube andthe outlet end of the tube on the duct may both be moved relative to theduct when the distance between the two ends are changed.

FIG. 10 depicts frequency and spin rate tracking using a flexible,constant length tube. For demonstration purposes, the acoustic mode isshown given an initial resonant frequency of 2000 Hz. The mode is thenslowed to 1800 Hz resulting in a the orientation angle increasing by12.2° from the original 63.8°.

The helix angle, θ as described in equation (3) and FIG. 3. increaseswith increasing axial wavenumber, k_(z). Therefore, as the frequencyincreases so too does the helix angle. For non-spinning modes, theflexible tubes of this present invention would be arranged parallel tothe duct axis and their inlet/outlet separation distance would be tunedto achieve the proper acoustic path delay. For spinning modes, thecircumferential array of the tubes of this present invention would eachbe aligned with the helix angle for best performance. Variations infrequency for a spinning mode will then require modifying both theinlet/outlet separation distance and the tube orientation angle of thetubes of this present invention to be adjusted allowing for continuousattenuation. The orientation angle of the tubes may be adjusted, forexample, using the tuning ring depicted in FIG. 7 or using another meansof adjustment.

As an example, for a tone at 2000 Hz having a mode order of (2,1) itsinflection point would be 3.05. By equation (5), and assuming a ductradius of a=0.2 m, k_(qp)=30.5 m⁻¹. For demonstration purposes, we letM=0 and by equation (4) the axial wavenumber is k_(z)=20.3 m⁻¹. For thecircumferential wavenumber of order 2, k_(s)=10 m⁻¹. Therefore, byequation (3), the helix angle is 63.8°. The HQ tubes must then bearranged at an angle of 90°−63.8°=26.2° relative to the duct axis asshown in FIG. 5.

Assuming now, the tone decreases from 2000 Hz to 1800 Hz, the axialwavenumber now decreases to 12.6 m⁻¹. The resulting helix angle thenchanges to 51.6° requiring the angle of the flexible HQ tubes of thispresent invention to change to 38.4° relative to the duct axis—adifference of 12.2°. This is illustrated in FIG. 6.

In addition, the inlet/outlet separation distance must also change asnoted previously in section I.2. Both the separation distance and theangle of the tubes of this present invention may be continually tuned asthe disturbance changes to allow for continuous maximum attenuation.

FIG. 11 depicts the placement of several constant length, flexible tubes201 with inlet and outlet tuning rings mounted in a turbofan engine 400.Arrays of constant length, flexible tubes 201 are shown at the fan inlet401 and the fan outlet 402. This embodiment uses microphones 403 tomeasure acoustic pressure which is used as the error signals for thecontroller, however other error signals, such as but not limited toacoustic intensity, may be used to determine the optimal separationdistance and angular orientation of each array of constant length,flexible tubes 201.

FIG. 12 depicts a detailed view of one constant length, flexible tube201 having an outlet tuning ring 310 and an inlet tuning ring 320mounted in a turbofan engine as identified in FIG. 11. The outlet tuningring 310 is used to vary the linear and angular orientation of the tubeoutlet 410. The inlet tuning ring 320 is used to vary the linear andangular orientation of the tube inlet 420.

A control system is envisioned for the tuning of the separation distanceand angular orientation of an array of constant length, flexible tubes201. A feed-back or feed-forward control system is used for the tuningof the separation distance and angular orientation of an array offlexible, constant length tubes 201. Such a control system wouldcomprise an error measurement, most likely but not limited to the use ofseveral microphones 403, a controller for computing the necessarycomputations, appropriate signal conditioning hardware and devices (411,421) used for the manipulation of the linear and angular orientations ofthe outlet tuning ring 310 and the inlet tuning ring 320.

Linear and angular adjustment of the outlet tuning ring 310 isaccomplished by sending the appropriate control signal from thecontroller to the output tuning ring adjustment device 411. The outputtuning ring adjustment device 411 actuates an appendage 412 connected tothe output tuning ring 310 thereby adjusting the linear and angularlocations of the outlets 410 of that respective array.

Linear and angular adjustment of the inlet tuning ring 320 isaccomplished by sending the appropriate control signal from thecontroller to the input tuning ring adjustment device 421. The inputtuning ring adjustment device 421 actuates an appendage 422 connected tothe input tuning ring 320 thereby adjusting the linear and angularlocations of the outlets 410 of that respective array.

FIG. 13 shows the placement of a single array of constant length,flexible tubes 201 mounted at the inlet 401 of a turbofan engine. Theacoustic energy radiating from the fan 404 approaches the inlets 320 ofthe constant length, flexible tubes 201. This acoustic energy thendiverges partly into the tube inlets 320 and into the array of constantlength, flexible tubes 201 with the remaining acoustic energy continuingto propagate through the inlet 401 duct of the turbofan engine. The twoacoustic paths converge at the outlets 310 of the constant length,flexible tubes 201 thereby minimizing the acoustic energy radiated fromthe inlet 401 of the operating turbofan engine.

The linear and angular orientations of the inlets 320 are varied bymeans of the inlet tuning ring 420. The linear and angular orientationof the outlets 310 are varied by means of the outlet tuning ring 410.See FIG. 14 for a detailed view.

FIG. 14 depicts a detailed view of a turbofan engine having one array ofconstant length, flexible tubes 201 with an outlet tuning ring 410 andan inlet tuning ring 420 as identified in FIG. 13.

The linear and angular orientations of the tube inlets 320 are varied bytranslating and rotating the inlet tuning ring 420. The orientation ofthe inlet tuning ring 420 is varied using the inlet tuning ringadjustment device 421 via an appendage 422.

The linear and angular orientations of the outlets 310 are varied bytranslating and rotating the inlet tuning ring 410. The orientation ofthe outlet tuning ring 410 is varied using the inlet tuning ringadjustment device 411 via an appendage 412.

FIG. 15 shows a duct 100 and the use of a branched, constant length,flexible tube system comprising of a constant length, flexible tube 501and a constant length, flexible tube branch 502. The tube systemcomprises of three ports, 510, 520 and 530. The linear and angularorientation of port 510 is varied using tuning ring 511. The linear andangular orientation of port 520 is varied using tuning ring 521. Thelinear and angular orientation of port 530 is varied using tuning ring531. An adjustable obstruction 550 prevents acoustic energy from flowingthrough the tube branch 502.

An acoustic wave 600 propagates through the duct 100 where it partlydiverts into the tube 501 with the remaining acoustic energy continuingthrough the duct 100 as acoustic wave 602. 602 exits the tube 501 atport 520 to combine with acoustic wave 602 in the duct forming acousticwave 603 having a lower acoustic pressure than acoustic wave 600.

The embodiment of a noise attenuation apparatus for ducts such asturbofan aircraft engines comprises at least one tube having an inletend and an outlet end, wherein the ends are separated by a distancecapable of being varied during operation, wherein the ends are in fluidcommunication with the duct. Furthermore, at least one flexible tubebranch attached in fluid communication to at least one end of the atleast one tube. There are at least three ports wherein the at leastthree ports comprises an inlet port at the inlet end of the at least onetube having an open position that allows the inlet end of the tube to bein fluid communication with the duct and a closed position to preventthe inlet end to be in fluid communication with the duct. Also includedis a branch port that controls fluid communication between the at leastone tube and the at least one branch tube. Lastly an outlet port at theoutlet end of the at least one tube having an open position that allowsthe inlet end of the tube to be in fluid communication with the duct anda closed position to prevent the inlet end to be in fluid communicationwith the duct. The apparatus may include a measurement system capable ofsensing the presence of spinning and non-spinning acoustic modes andtheir respective amplitudes, phase, resonant frequencies and angles ofpropagation. A control system may be used to control the fluidcommunication of the at least one tube with the duct. Further optionsinclude an outlet tuning ring capable of rotation around the axis of theduct and movement along the length of the duct. The embodiment caninclude an outlet tuning ring capable of rotation around the axis of theduct and movement along the length of the duct wherein the inlet endfixed relative to the duct and the outlet end of the tube is affixed tothe outlet tuning ring.

FIG. 16 shows a duct 100 and the use of a branched, constant length,flexible tube system comprising of a constant length, flexible tube 501and a constant length, flexible tube branch 502. The tube systemcomprises of three ports, 510, 520 and 530. The linear and angularorientation of port 510 is varied using tuning ring 511. The linear andangular orientation of port 520 is varied using tuning ring 521. Thelinear and angular orientation of port 530 is varied using tuning ring531. An adjustable obstruction 551 prevents acoustic energy from flowingthrough the port 520 thus diverting any flow through tube 501 throughtube branch 502.

An acoustic wave 600 propagates through the duct 100 where it partlydiverts 601 into the tube 501 with the remaining acoustic energycontinuing through the duct 100 as acoustic wave 602. Acoustic wave 601is prevented from exiting the tube 501 at port 520 by an obstruction 551and instead is diverted through tube branch 502. Upon exiting tubebranch 502 through port 530 acoustic wave 601 combines with acousticwave 602 forming acoustic wave 603 having a lower acoustic pressure thanacoustic wave 600.

FIG. 17 shows a duct 100 and the use of a branched, constant length,flexible tube system comprising of a constant length, flexible tube 501and a constant length, flexible tube branch 502. The tube systemcomprises three ports 510, 520 and 530. The linear and angularorientation of port 510 is varied using tuning ring 511. The linear andangular orientation of port 520 is varied using tuning ring 521. Thelinear and angular orientation of port 530 is varied using tuning ring531. The adjustable obstructions 550 (FIG. 15) and 551 (FIG. 16) areopen allowing acoustic energy to flow through all portions of the HQtube 501 and the tube branch 502. Such a design allows for one apparatuscapable of attenuating three acoustic modes of different frequencycontent and angle of propagation. The three path length/separationdistance combinations are 1) port 510 to port 520, 2) port 520 to port530 and port 510 to port 530.

An acoustic wave 600 propagates through the duct 100 where it partlydiverts as acoustic wave 601 into the tube 501 with the remainingacoustic energy continuing through the duct 100 as acoustic wave 602.

At the junction of the tube and the tube branch acoustic wave 601diverts partly into acoustic wave 605 exiting through port 520.

Acoustic wave 602 in the duct 100 diverts partly as acoustic wave 604into the tube through port 520 where it then combines with remainingacoustic energy of 601 less the component of 605 forming then acousticwave 606 traveling through the tube branch 502.

Acoustic wave 605 exits the HQ tube through port 520 into the duct 100where it combines with the portion of acoustic wave 602 less acousticwave 604 thus forming acoustic wave 607 in the duct 100.

Acoustic wave 606 then travels through the tube branch 502 untilcombining with acoustic wave 607 in the duct 100 forming acoustic wave608 having a lower acoustic pressure than acoustic wave 600.

The above invention describes a method of noise attenuation for ductssuch as turbofan aircraft engines comprising the steps of providing atleast one tube having an inlet end and an outlet end, wherein the endsare separated by a distance capable of being varied during operation,wherein the ends are in fluid communication with the duct and the tubehas a fixed length. The next step is measuring the frequency of thesound waves of the duct. Then actuating with a distance actuator forchanging the distance between the inlet end and the outlet end of thetube within the duct to reduce noise. The method involves themanipulation of the above mentioned parts described throughout thespecification based upon the application of the calculations describedherein in a computer that actuates controllers to dynamically adjust theposition and orientations of the tubes throughout the range of theoperation of the duct to minimize unwanted noise.

The noise may be reduced further by moving at least one tuning ringcapable of rotation around the axis of the duct and movement along thelength of the duct wherein the end of the at least one tube is attachedto the inlet tuning ring to reduce noise. The method can includeproviding a turbofan aircraft engine having a duct.

The invention has been described in terms of the several embodiments. Itis to be understood that the preceding description is given toillustrate various embodiments of the present inventive concepts. Thespecific examples are not to be considered as limiting, except inaccordance with the following claims.

1. A noise attenuation apparatus for ducts such as a turbofan aircraftengine duct comprising: at least one flexible fixed length tube havingan inlet end and an outlet end, wherein the ends are separated by adistance capable of being varied during operation, wherein the ends arein fluid communication with the duct; and a distance actuator forchanging the distance between the inlet end and the outlet end of thetube within the duct.
 2. The apparatus of claim 1 further comprising: anorientation actuator, wherein the tube has an orientation angle that canbe changed by the orientation actuator.
 3. The apparatus of claim 1further comprising: a measurement system connected to the duct capableof sensing the presence of spinning and non-spinning acoustic modes andtheir respective amplitudes, phase, resonant frequencies and angles ofpropagation.
 4. The apparatus of claim 3 further comprising: a controlsystem to control the separation distance of the inlet end and theoutlet end of the at least one tube.
 5. The apparatus of claim 4 furthercomprising at least one microphone positioned near the duct to measureacoustic pressure in the duct and to send signals to the controllercorresponding to the acoustic pressure measurements.
 6. The apparatus ofclaim 1 further comprising: an outlet tuning ring attached to the outletend and capable of rotation around the axis of the duct and movementalong the length of the duct.
 7. The apparatus of claim 1 wherein theposition of the inlet end of the tube is fixed relative to the duct. 8.The apparatus of claim 1 wherein the position of the outlet end of thetube is fixed relative to the duct.
 9. The apparatus of claim 1 whereinthe distance between the inlet end of the tube and the outlet end of thetube on the duct are both moved relative to the duct when the distancebetween the two ends is changed.
 10. The apparatus of claim 1 furthercomprising: an outlet tuning ring capable of rotation around the axis ofthe duct and movement along the length of the duct wherein the inlet endis fixed relative to the duct and the outlet end of the tube is affixedto the outlet tuning ring.
 11. The apparatus of claim 10 furthercomprising an outlet tuning ring adjustment device connected to theoutlet tuning ring for controlling movement of the outlet tuning ring toadjust the location of the outlet end.
 12. The apparatus of claim 1further comprising: an inlet tuning ring capable of rotation around theaxis of the duct and movement along the length of the duct wherein theoutlet end of the at least one tube is fixed relative to the duct andthe inlet end of the tube is affixed to the inlet tuning ring.
 13. Theapparatus of claim 12 further comprising an inlet tuning ring adjustmentdevice connected to the inlet tuning ring for controlling movement ofthe inlet tuning ring to adjust the location of the inlet end.
 14. Theapparatus of claim 1 further comprising: an inlet tuning ring capable ofrotation around the axis of the duct and movement along the length ofthe duct wherein the inlet end of the at least one tube is attached tothe inlet tuning ring; and, an outlet tuning ring capable of rotationaround the axis of the duct and movement along the length of the ductwherein the outlet end of the at least one tube is attached to theoutlet tuning ring.
 15. The apparatus of claim 1 further comprising: aturbofan aircraft engine having a duct.
 16. A method of noiseattenuation for ducts such as a turbofan aircraft engine duct comprisingthe steps of: providing at least one tube having an inlet end and anoutlet end, wherein the ends are separated by a distance capable ofbeing varied during operation, wherein the ends are in fluidcommunication with the duct and the tube has a fixed length; andmeasuring the frequency of the sound waves of the duct; actuating with adistance actuator for changing the distance between the inlet end andthe outlet end of the tube within the duct to reduce noise.
 17. Themethod of claim 16 further comprising the steps of: moving at least onetuning ring capable of rotation around the axis of the duct and movementalong the length of the duct wherein the end of the at least one tube isattached to the inlet tuning ring to reduce noise.
 18. The method ofclaim 16 further comprising the steps of: providing a turbofan aircraftengine having a duct.
 19. The apparatus of claim 1 further comprising:an inlet tuning ring connected to the inlet end and capable of rotationaround the axis of the duct and movement along the length of the duct;an inlet tuning ring adjustment device connected to the inlet tuningring for controlling movement of the inlet tuning ring to adjustlocation of the inlet end; an outlet tuning ring connected to the outletend and capable of rotation around the axis of the duct and movementalong the length of the duct; and, an outlet tuning ring adjustmentdevice connected to the outlet tuning ring for controlling movement ofthe outlet tuning ring to adjust location of the outlet end.
 20. A noiseattenuation apparatus for ducts such as a turbofan aircraft engine ductcomprising: an engine duct having an inlet and an outlet; a plurality offlexible, fixed length tubes each having an inlet end and an outlet end,wherein the ends are separated by a distance capable of being variedduring operation, wherein interiors of the tubes are in communicationwith an interior of the duct, and wherein the tubes are positioned onthe duct in patterns selected from the group consisting of helical,circumferential and mixtures thereof, and in locations selected from thegroup consisting of the duct inlet, the duct outlet and mixturesthereof; and, at least one distance actuator for changing the distancebetween the inlet end and the outlet end of at least one of theplurality of tubes.